initial commit

1 files changed, 72 insertions, 0 deletions

diff --git a/euler102.c b/euler102.c
new file mode 100644
index 0000000..272a54a
--- /dev/null
+++ b/euler102.c
@@ -0,0 +1,72 @@
+#include <stdio.h>
+#include <stdbool.h>
+
+typedef struct
+{
+	int a[2],b[2],c[2];
+} triangle;
+
+static inline void
+vsub(int d[], int s[], int t[])
+{
+	d[0] = s[0] - t[0];
+	d[1] = s[1] - t[1];
+}
+
+static int
+ext_product(int s[], int t[])
+{
+	return s[0] * t[1] - s[1] * t[0];
+}
+
+static int sign(int x)
+{
+	if (x < 0)
+		return -1;
+	if (x > 0)
+		return 1;
+	return 0;
+}
+
+bool o_in_tri(triangle *t)
+{
+	int ab[2], ac[2], bc[2];
+
+	vsub(ab, t->b, t->a);
+	vsub(ac, t->c, t->a);
+	vsub(bc, t->c, t->b);
+
+	/* sig(ABxAC) = sig(OBxOC) */
+	int sabc = sign(ext_product(ab, ac));
+	int sobc = sign(ext_product(t->b, t->c));
+	if (sabc != sobc)
+		return false;
+
+	/* sig(BAxBC) = sig(OAxOC) */
+	int sbac = sign(ext_product(bc, ab));
+	int soac = sign(ext_product(t->a, t->c));
+	if (sbac != soac)
+		return false;
+
+	/* sig(CAxCB) = sig(OAxOB) */
+	int scab = sign(ext_product(ac, bc));
+	int soab = sign(ext_product(t->a, t->b));
+	if (scab != soab)
+		return false;
+
+	return true;
+}
+
+int main()
+{
+	triangle t;
+	int n = 0;
+	/* the data uses ',' to split the integers, process it with
+	 * sed 's/,/ /g' first. */
+	while (scanf("%d%d%d%d%d%d", &t.a[0], &t.a[1], &t.b[0], &t.b[1],
+				&t.c[0], &t.c[1]) != EOF) {
+		if (o_in_tri(&t))
+			n++;
+	}
+	printf("%d\n", n);
+}